The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. OTO is therefore an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are 8-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA framework type, such as SAPO-34.
CHA framework-type molecular sieves appear to be particularly suitable catalysts for the OTO reaction since they have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusive transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that increasing the silica to alumina molar ratio seems to be a key requirement in the use of CHA framework-type aluminosilicates in OTO reactions.
Chabazite is a naturally occurring zeolite with the approximate formula Ca6Al12Si24O72. Three synthetic forms of chabazite are described in “Zeolite Molecular Sieves”, by D. W. Breck, published in 1973 by John Wiley & Sons, the complete disclosure of which is fully incorporated herein by reference. The three synthetic forms reported by Breck are Zeolite “K-G”, described in J. Chem. Soc., pg. 2822 (1956), Barrer et al; Zeolite D, described in British Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181 (1962). Zeolite K-G zeolite has a silica:alumina mole ratio of 2.3:1 to 4.15:1, whereas zeolites D and R have silica:alumina mole ratios of 4.5:1 to 4.9:1 and 3.45:1 to 3.65:1, respectively. The relatively low silica to alumina molar ratio of these materials makes them less than optimal as catalysts for OTO reactions.
Considerable work has therefore been conducted on the synthesis of CHA framework-type aluminosilicate molecular sieves having high silica to alumina molar ratios and in particular with silica to alumina molar ratios greater than 15:1, preferably greater than 100:1.
For example, U.S. Pat. No. 4,544,538 describes the synthesis of a synthetic form of chabazite-type aluminosilicate, SSZ-13, using N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantylammonium cations and/or N,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventional OH− medium. According to the '538 patent, SSZ-13 typically has a silica to alumina molar ratio of 8 to 50 but it is stated that higher molar ratios can be obtained by varying the relative ratios of the reactants in the synthesis mixture and/or by treating the zeolite with chelating agents or acids to remove aluminum from the zeolite lattice. However, attempts to synthesize SSZ-13 in OH− media at silica to alumina molar ratios in excess of 100 have been unsuccessful and have produced ITQ-1 or SSZ-23, depending on the alkali metal cation present. Moreover, increasing the silica to alumina molar ratio of SSZ-13 by dealumination has met little or no success.
U.S. Pat. No. 6,709,644 describes a zeolite that is identified as SSZ-62 and has a CHA framework type and a crystal size of 0.5 microns or less. SSZ-62 is said to have a silica to alumina molar ratio in excess of 10, such as in excess of 30, but the only synthesis example produces a material with a silica to alumina molar ratio of 22. Synthesis is effected in a hydroxyl medium in the presence of N,N,N-trimethyl-1-adamantammonium cation as the structure directing agent. The zeolite can be steamed, purportedly to help stabilize the crystalline lattice to attack from acids.
An all silica crystalline molecular sieve having the CHA framework type has been hydrothermally synthesized using N,N,N-trimethyladamantylammonium in hydroxide form as the structure-directing agent, but the synthesis requires the presence of concentrated hydrofluoric acid. See Diaz-Cabanas, M-J, Barrett, P. A., and Camblor, M. A. “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density”, Chem. Commun. 1881 (1998).
More recently, an aluminosilicate with the CHA framework type and having a silica to alumina molar ratio in excess of 100, such as from 150 to 2000, has been synthesized in the presence of fluoride ions. See U.S. Patent Application Publication No. 2003/0176751, published Sep. 18, 2003. Structure directing agents employed include N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantammonium cations and N,N,N-trialkyl-exoaminonorbornane.
U.S. Published Patent Application No 2005/0154244, published Jul. 14, 2005, discloses a crystalline material comprising a CHA framework-type molecular sieve with stacking faults or at least one intergrown phase of a CHA framework-type molecular sieve and an AEI framework-type molecular sieve, wherein the material is substantially free of framework phosphorus and has a composition involving the molar relationship (n)X2O3:YO2 wherein X is a trivalent element, Y is a tetravalent element and n is from 0 to about 0.5. The material can be synthesized using a mixed directing agent comprising an N,N,N-trialkyl-1-adamantylammonium compound and an N,N-diethyl-2,6-dimethylpiperidinium compound, normally in the presence of fluoride ions.
In pending US 2007-0100185 A1, we have described a method of synthesizing a crystalline material comprising a CHA framework-type molecular sieve, the method comprising: a) forming a reaction mixture capable of forming said crystalline material, wherein the reaction mixture comprises a structure directing agent having the formula:[R1R2R3N—R4]+Q−wherein R1 and R2 are independently selected from hydrocarbyl alkyl groups and hydroxy-substituted hydrocarbyl groups having from 1 to 3 carbon atoms, provided that R1 and R2 may be joined to form a nitrogen-containing heterocyclic structure, R3 is an alkyl group having 2 to 4 carbon atoms and R4 is selected from a 4- to 8-membered cycloalkyl group, optionally, substituted by 1 to 3 alkyl groups each having from 1 to 3 carbon atoms, and a 4- to 8-membered heterocyclic group having from 1 to 3 heteroatoms, said heterocyclic group being, optionally, substituted by 1 to 3 alkyl groups each having from 1 to 3 carbon atoms and each heteroatom in said heterocyclic group being selected from the group consisting of O, N, and S, or R3 and R4 are hydrocarbyl groups having from 1 to 3 carbon atoms joined to form a nitrogen-containing heterocyclic structure; and Q− is an anion; and b) recovering from said reaction mixture said crystalline material comprising a CHA framework-type molecular sieve.
Also, in pending US 2007-0100185 A1, we have described a method of synthesizing a crystalline material comprising a CHA framework-type molecular sieve and having a composition involving the molar relationship:(n)X2O3:YO2 wherein X is a trivalent element; Y is a tetravalent element; and n is from 0 to less than 0.01, such as from about 0.0005 to about 0.007, the method comprising: (a) preparing a reaction mixture capable of forming said material, said mixture comprising a source of water, a source of an oxide of a tetravalent element Y and, optionally, a source of an oxide of a trivalent element X, wherein the reaction mixture is substantially free of fluoride ions added as HF; (b) maintaining said reaction mixture under conditions sufficient to form crystals of said crystalline material; and (c) recovering said crystalline material.
Despite these synthesis advances, many of the resultant high silica CHA framework-type molecular sieves, and particularly those synthesized in the absence of HF, exhibit reduced MTO performance, such as an unexpectedly high selectivity to undesirable coke and methane. According to the invention, it has now been found that the MTO performance, and most importantly the prime olefin selectivity, of high silica CHA framework-type molecular sieves can be enhanced by mild steam treatment. Whereas the reason for this result is not fully understood, recent work has shown that many as-synthesized high silica CHA zeolites exhibit framework defects and it is believed that the steam treatment serves to heal these defects.
U.S. Pat. No. 4,326,994 discloses a method for increasing the catalytic activity of an acid zeolite having a determinable initial activity and characterized by a silica to alumina mole ratio of at least 12 and a constraint index within the approximate range of 1 to 12. The zeolite is selected from the group consisting of ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, and ZSM-38 and the activation method comprises contacting said zeolite with water for a sufficient treating time, temperature, and water partial pressure wherein said time, temperature and pressure is represented by the following relationship of treating time and water pressure at constant temperatures:0.001(Pt)T<(Pt)<10(Pt)T where
(Pt)T=2.6×10−9 e 16000/T
P=Water Partial Pressure, atmosphere
t=Treating Time, Hours
T=Temperature, ° K.
U.S. Pat. No. 5,095,163 discloses a method of hydrothermal treatment of silicoaluminophosphate molecular sieves, such as SAPO-34, at temperatures in excess of about 700° C. for periods sufficient to destroy a large proportion of their acid sites while retaining at least 80 percent of their crystallinity. The hydrothermal treatment is found to result in a catalyst for converting methanol to lower olefins having increased catalyst life, increased selectivity for C2 to C3 olefins and decreased selectivity for paraffin production than the untreated SAPO-n starting composition.
Steaming is also known to be effective in the stabilization of large pore zeolites, such as zeolite Y. For example, U.S. Pat. No. 3,493,519 teaches a method of preparing ultrastable zeolite Y in which an ammonium form of zeolite Y is heated in the presence of rapidly flowing steam, and the resultant steamed product is base-exchanged with an ammonium salt and then treated with a chelating agent capable of combining with aluminum at a pH between 7 and 9. The steaming results in movement of mobile silicate species to defect sites, which is only possible with relatively large pore molecular sieves such as zeolite Y.